When I looked into this a couple of years ago the most efficient (in terms of power consumption) were the EvaKool units built by an Australian company for RVs (Caravans) used in the Outback. Unfortunately I couldn't find any way of getting one imported into the US.
EvaKoolI do a lot of dry camping and opted for a portable Engle 12/1220v refer/freezer that seems to have the lowest power consumption for portables. Even so, with my 300w of solar and two 12v Lithium Ion 125ah batteries I can only go about 3-4 days before having to top off the batteries with the generator.